In recent years, power generation using a wind turbine has been developed as clean energy. Blades of the wind turbine rotate around a shaft with wind energy; torque of the blades is converted into electric power to obtain generation output power.
The generation output power of the wind turbine is equal to a product of shaft-end output power (output power generated by the blade) and conversion efficiency (efficiency of a bearing or a generator). Further, the shaft-end output power is expressed by the following expression; a blade having high blade efficiency and a large blade diameter increases power generation.Shaft-end output power=1/2×air density×wind speed3×blade efficiency×π×(blade diameter/2)2 
The blade efficiency has a theoretical upper limit (Betz limit=0.593), and an actual upper limit is about 0.5 due to an influence of a wake and air resistance of the blade. Thus, it is difficult to make further significant increase in the blade efficiency.
Meanwhile, since the square of the blade diameter influences output power, it is effective to enlarge the blade diameter to increase power generation. However, enlargement of the blade diameter increases aerodynamic load (a thrust force applied in an inflow direction and moment transferred to a blade root), which may increase sizes or weights of devices such as a rotor head, a nacelle, and a tower, and thus increase cost. Thus, a technique is needed to increase a length of a blade while preventing an increase in aerodynamic load of the blade. As an aerodynamically (in terms of a blade shape) conceivable method to avoid an increase in load, there is a method of reducing a chord length (blade chord length) (specifically, increasing an aspect ratio or reducing solidity) to reduce a blade projection area and reduce aerodynamic load.
The aspect ratio and the solidity are expressed by the following expression.Aspect ratio=blade length2/blade projection area  (1)Solidity=entire blade projection area/blade sweep area=(the number of blades×average chord length)/(n×(blade diameter/2)2)  (2)
Generally, a wind turbine blade has a predetermined optimum chord length for a predetermined tip speed ratio, and has a relationship in the following expression (Wind Energy Handbook, John Wiley & Sons, p378).Copt/R×λ2×λ2CLdesign×r/R≈16/9×π/n  (3)
where Copt is an optimum chord length, R (blade radius) is ½ of the blade diameter, λ is a design tip speed ratio, CLdesign is design lift coefficient, r is a radial position of a blade section, and n is the number of blades.
The design tip speed ratio is blade tip peripheral speed/infinite upstream wind speed. The design lift coefficient is a lift coefficient at an angle of attack where a lift-drag ratio (lift/drag) of a blade profile (blade section) is maximum, and determined by an (aerodynamic) shape of the blade profile (blade section) and an inflow condition (Reynolds number).
FIG. 8 shows definition of the Reynolds number used herein. As shown in FIG. 8, the Reynolds number of the wind turbine considers a relative wind speed on a predetermined section A-A of a blade rotated at a predetermined number of revolutions, and expressed by the following expression.Reynolds number=air density×relative wind speed on blade section×chord length of blade section/viscosity coefficient of air
PTL 1 mentioned below discloses a blade profile for increasing output power of a wind turbine. Specifically, a blade profile is disclosed in which a blade thickness ratio is within a range of 14% to 45% and a design lift coefficient is within a range of 1.10 to 1.25. (See claim 1.)